A conventional holographic memory system usually employs a page-oriented storage approach. An input device such as an SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as a CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-oriented storage approach. All of these systems, however, suffer from a common drawback in that they require the recording of a huge number of separate holograms in order to fill the memory to capacity. A typical page-oriented storage system using a megabit-sized array would require a recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the holographic exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, a conventional holographic recording apparatus for use in a conventional holographic ROM system such as the one shown in FIG. 1 has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.
The conventional holographic recording apparatus shown in FIG. 1 includes a light source 10, half wave plates (HWPs) 12, 22, a polarizing beam splitter (PBS) 14, mirrors 16, 24, 26, a conical mirror 18, a hologram medium 20, an expanding unit 28, and a mask 30.
The light source 10 emits a laser beam of a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is consisted of only one type of linear polarization, e.g., either P-polarization or S-polarization, is provided to the HWP 12. The HWP 12 rotates the polarization of the laser beam by θ degree (preferably 45°) And then, the polarization-rotated laser beam is fed to the PBS 14.
The PBS 14, which is manufactured by repeatedly depositing at least two kinds of materials each of which has a different refractive index, serves to transmit one type of polarized laser beam, e.g., P-polarized beam, and reflect the other type of the polarized laser beam, e.g., S-polarized beam. Thus the PBS 14 divides the polarization-rotated laser beam into a transmitted laser beam (hereinafter, a reference beam) and a reflected laser beam (hereinafter, a signal beam) having different polarizations, respectively.
The reference beam, e.g., of a P-polarization, is reflected by the mirror 16, and the reference beam is projected onto the conical mirror 18, the conical mirror 18 being of a circular cone mirror having a circular base with a preset base angle between the circular cone and the circular base. The reference beam is reflected once more by the conical mirror 18 to propagate toward the hologram medium 20. The incident angle of the reference beam on the hologram medium 20 is determined by the base angle of the conical mirror 18. The geometry of the circular cone mirror is specified in order that the incident angle of the reference beam is constant at all positions on the hologram medium 20.
On the other hand, the signal beam, i.e., of an S-polarization, is projected to the HWP 22. The HWP 22 converts the polarization of the signal beam such that the polarization of the signal beam becomes identical to that of the reference beam. The signal beam is sequentially reflected by the mirrors 24 and 26, so that the signal beam may be fed to the expanding unit 28. The expanding unit 28 expands a beam size of the signal beam so as to make it have a suitable dimension relative to the mask 30 and the hologram medium 20. The signal beam is preferably a collimated beam which has planar wavefronts that are perpendicular to their direction of propagation. The signal beam is projected to the mask 30. The mask 30 has an opaque film deposited on a transparent plate, wherein the opaque film has a data pattern, e.g., spiral tracks with a predetermined track pitch. Hundreds to thousands of digital data to be recorded are embedded in a shape of a sequence of, e.g., slits, on the spiral tracks. Specifically, the digital data constituted with binary bits may be marked on a bit-by-bit basis on the spiral tracks in the opaque film. The collimated signal beam of, e.g., a normally incident plane wave, is modulated with the digital data embedded on the spiral tracks of the mask 30 so that the modulated signal beam is projected onto the hologram medium 20.
The hologram medium 20 is, e.g., a disk-shaped material for recording the data patterns. Specifically, the reference beam and the modulated signal beam interfere with each other within the hologram medium 20 so that the interference pattern between the reference beam and the modulated signal beam may be recorded as a hologram in the hologram medium 20.
Referring to FIG. 2, there is shown a block diagram to illustrate a conventional apparatus for reconstructing the hologram without a focusing servo mechanism. The apparatus includes a light source 50, a reducing unit 52, a mirror 54, a motor 55, the hologram medium 20 coated with a coating film 56, first and second lenses 58, 62, a pinhole plate 60 and a detector 64. The hologram medium 20 has included the interference patterns created by the modulated signal beam and the reference beam which are coherent with each other as described above.
The modulated signal beam may be reconstructed by illuminating the interference patterns with a reconstructing beam, of the same wavelength but with wavefronts that are “complex conjugate” (the reverse wavefront and the reverse direction) to the wavefronts in the reference beam. In other words, the light source 50 generates a laser beam which is the complex conjugate of the reference beam. The laser beam is provided to the reducing unit 52, in which the beam size of the laser beam is reduced to a predetermined size, i.e., 100 μm. The reduced laser beam is reflected by the mirror 54 and then provided into the hologram medium 20 as the reconstructing beam.
Since the reconstructing beam, being the complex conjugate of the reference beam, propagates in the reverse direction of the reference beam, the interference patterns impart a reconstructed signal beam that is identical to the complex conjugate of the modulated signal beam. Therefore, the reconstructed signal beam appears to be released from the interference patterns in “reverse” to the modulated signal beam as shown in FIG. 2.
The reconstructed signal beam is introduced through the first lens 58, the pinhole plate 60 and the second lens 62 to the detector 64. Specifically, the reconstructed signal beam is diffracted with a diffraction angle θ and then converged by the first lens 58 to the pinhole plate 60. Since the track pitch between two neighboring tracks on the spiral tracks in the hologram medium 20 is at most several μm, a number of tracks may be illuminated with the reconstructed signal beam of 100 μm in diameter so that a number of tracks may be simultaneously reconstructed as the reconstructed signal beam. Referring to FIG. 3, there is shown a plan view of an exemplary pinhole plate 60 with a pinhole 60a, wherein a pinhole width S of the pinhole 60a in the pinhole plate 60 corresponds to a track width of each spiral track itself in the hologram medium 20. Through the pinhole 60a in the pinhole plate 60, only a portion corresponding to each spiral track among the reconstructed signal beam may be transmitted. The reconstructed signal beam transmitted through the pinhole 60a may be diffracted once more and converged by the lens 62 to the detector 64.
In order that the detector 64 detects the reconstructed signal beam precisely, a focusing servo between the detector 64 and the hologram medium 20 must be operated. Referring to FIGS. 4A to 4C, there are shown an inside, a normal and an outside focusing at the pinhole 60a of the pinhole plate 60, respectively. In case of the inside focusing shown in FIG. 4A, the reconstructed signal beam to be transmitted through the pinhole 60a is so weak that the pinhole plate 60 as well as the detector 64 must be controlled to move near to the hologram medium 20 to obtain a normal focusing. Also, in case of the outside focusing shown in FIG. 4C, the reconstructed signal beam to be transmitted through the pinhole 60a is so weak that the pinhole plate 60 as well as the detector 64 must be controlled to move away from the hologram medium 20 to obtain a normal focusing. Since, however, both the inside and the outside focusing are not different from each other in that the reconstructed signal beam is so weak, a focusing servo signal to control the distance between the pinhole 60a of the pinhole plate 60 and the hologram medium 20 or the distance between the detector 64 and the hologram medium 20 cannot be obtained under the conventional holographic ROM system described above.